1. Field of the Invention
This invention relates to the prevention of gas hydrate blockage in oil and natural gas pipelines containing low-boiling hydrocarbons and water. More specifically, the invention relates to a method of controlling gas hydrate blockage through the addition of various chemical compositions.
2. Background of the Related Art
When hydrocarbon gas molecules dissolve in water, the hydrogen-bonded network of water molecules encapsulates the gas molecules to form a cage-like structure or hydrate. Higher pressures and lower temperatures foster the formation of these structures. These hydrates grow by encapsulating more and more gaseous molecules to form a crystalline mass. The crystalline mass agglomerates to form a larger mass that can result in a plugged transmission line. The hydrocarbon gases that form the majority of the hydrates include methane, ethane, propane, n-butane, iso-butane, n-pentane, iso-pentane, and combinations of these gases.
Thermodynamic hydrate inhibitors, such as methanol or one of the glycols, have traditionally been used to prevent these hydrate formations. These thermodynamic inhibitors are effective at 5-30% (or higher) based on the amount of water. As oil companies are exploring new production in deep waters, the total gas/oil/water productions are also increasing. The use of these thermodynamic inhibitors is not viable in these applications due to logistics.
Kinetic hydrate inhibitors have been identified to prevent these hydrate formations so that the fluids can be pumped out before a catastrophic hydrate formation occurs. The kinetic inhibitors prevent or delay hydrate crystal nucleation and disrupt crystal growth. These kinetic hydrate inhibitors contain moieties similar to gas molecules previously mentioned. It is suspected that these kinetic inhibitors prevent hydrate crystal growth by becoming incorporated into the growing hydrate crystals, thereby disrupting further hydrate crystal growth. The growing hydrate crystals complete a cage by combining with the partial hydrate-like cages around the kinetic hydrate inhibitor moieties containing gas-like groups. These inhibitors are effective with or without the presence of a liquid hydrocarbon phase, but they are typically less effective in preventing the hydrate formation as the production pressure increases. Examples of kinetic hydrate inhibitors include poly(N-methylacrylamide), poly(N,N-dimethylacrylamide), poly(N-ethylacrylamide), poly(N,N-diethylacrylamide), poly(N-methyl-N-vinylacetamide), poly(2-ethyloxazoline), poly(N-vinylpyrrolidone), and poly(N-vinylcaprolactam).
Unlike the kinetic hydrate inhibitors, anti-agglomerate hydrate inhibitors are effective only in the presence of an oil phase. These inhibitors do not inhibit the formation of gas hydrates to the same level as kinetic inhibitors, rather their primary activity is in preventing the agglomeration of hydrate crystals. The oil phase provides a transport medium for the hydrates which are referred to as hydrate slurries so that the overall viscosity of the medium is kept low and can be transported along the pipeline. As such, the hydrate crystals formed in the water-droplets are prevented from agglomerating into a larger crystalline mass.
Examples of several chemicals acting as anti-agglomerate hydrate inhibitors have been reported in U.S. Pat. Nos. 5,460,728; 5,648,575; 5,879,561; and 6,596,911. These patents teach the use of quaternary ammonium salts having at least three alkyl groups with four or five carbon atoms and a long chain hydrocarbon group containing 8-20 atoms. Exemplary compositions include tributylhexadecylphosphonium bromide and tributylhexadecylammonium bromide.
More specifically, Klomp (U.S. Pat. No. 5,460,728) teaches the use of alkylated ammonium, phosphonium or sulphonium compounds having three or four alkyl groups in their molecule, at least three of which are independently chosen from the group of normal or branched alkyls having four to six carbon atoms. Klomp (U.S. Pat. No. 5,648,575) teaches very similar compositions having three or four organic groups in their molecule, at least three of which have at least four carbon atoms, i.e., two normal or branched alkyl groups having at least four carbon atoms and with a further organic moiety containing a chain of at least four carbon atoms. Klomp (U.S. Pat. No. 5,879,561) teaches the use of alkylated ammonium or phosphonium compounds having four alkyl groups, two of which are independently normal or branched alkyls having four or five carbon atoms and two more of which independently represent organic moieties having at least eight carbon atoms.
Klug (U.S. Pat. No. 6,369,004 B1) teaches the kinetic inhibition of gas hydrate formation using polymers based on reacting maleic anhydride with one or more amines. These polymers can also be used together with various other substances, called synergists, including tetrabutylammonium salts, tetrapentylammonium salts, tributylamine oxide, tripentylamine oxide, zwitterionic compounds having at least one butyl or pentyl group on the quaternary ammonium nitrogen atom, such as as Bu3N+—CH2—COO−. However, kinetic inhibitors are not effective as the pipeline pressure increases.
Rabeony (U.S. Pat. No. 6,015,929) teaches the use of specific zwitterionic compounds such as R(CH3)2N+—(CH2)4—SO3− as anti-agglomerate hydrate inhibitors. The synthesis of this product involves the use of butyl sultone.
However, there remains a need for hydrate inhibitor compounds that effectively prevent agglomeration of hydrates in oil and gas transportation and handling processes. It would be desirable to identify hydrate inhibitor compounds that are effective at higher pressures and/or lower temperatures such as those encounter in deep water production. It would be even more desirable if the same compounds had increased biodegradability.